Electronic mems device comprising a chip bonded to a substrate and having cavities and manufacturing process thereof

ABSTRACT

An electronic MEMS device is formed by a chip having with a main face and bonded to a support via an adhesive layer. A cavity extends inside the chip from its main face and is closed by a flexible film covering the main face of the chip at least in the area of the cavity. The support has a depressed portion facing the cavity and delimited by a protruding portion facing the main face of the chip. Inside the depressed portion, the adhesive layer has a greater thickness than the projecting portion so as to be able to absorb any swelling of the flexible film as a result of the expansion of the gas contained inside the cavity during thermal processes.

BACKGROUND

1. Technical Field

The present disclosure relates to an electronic micro-electro-mechanicalsystem (MEMS) device comprising a chip bonded to a substrate and havingcavities and to the manufacturing process thereof. In particular, thepresent disclosure applies to chemical sensors for detecting odorousmatters.

2. Description of the Related Art

As is known, in devices for recognizing odorous matters (such as the onedescribed, for example, in U.S. patent application Ser. No. 12/648,996,published as U.S. Application Publication No. 2010/0170324, andincorporated herein by reference in its entirety) the semiconductormaterial chip accommodates one or more cavities delimiting respectivemembranes carrying, i.a., respective adsorbent layers. Each membrane andthe respective adsorbent layer form part of an oscillating circuithaving electrical characteristics that vary with the weight of theensemble including the membrane, the adsorbent layer, and any possibleadsorbed material, enabling detection and possibly the amount of theadsorbed material.

Membranes may be manufactured using various techniques, some of whichenvisage formation of cavities extending on one side of the chip. Inthis case, when the chip is bonded on a substrate, the cavities are tobe kept clean without being contaminated with glue or any othermaterial.

FIGS. 1-3 show, for example, a chip 1 fixed to a substrate 10. The chip1 has a plurality of cavities 2, each delimiting at the top an ownmembrane 4 formed in the same monolithic semiconductor material as thechip 1. When the chip 1 forms a sensor for detecting odorous matters orin general a device for detecting chemical matters, each cavity mayhave, for example, a width of 250-300 μm and a depth of 500 μm, and thecavities may be made arranged at a mutual distance of 150-400 μm. Inthis case, the area of the membranes forms a sensitive region 5, andelectrodes and other sensitive layers, such as adsorbent layers (notshown), are formed on top of the membranes 3, whereas a circuitry area6, shown only schematically in FIG. 1 and including electroniccomponents, extends on the side of the sensitive area 5, for example,according to U.S. patent application Ser. No. 12/648,996.

An adhesive layer or film 7, for example a die-attach film, is appliedto the rear side of the chip 1 in order to close the cavities 2 at thebottom and prevent contamination thereof. The adhesive film 7 may belaminated on the back and cured using a thermal process so as to sealand protect the cavities.

A glue layer 8 fixes the chip 2, through the adhesive film 7, to thesubstrate 10. The substrate 10 may be of any type; for example, it maybe formed by a printed-circuit board, comprising a core region 11overlaid by at least one conductive layer 12, typically a copper layer,covered by a dielectric layer 13, typically a solder-mask layer. Theconductive layer 12 is shaped so as to form conductive regions andconnections, as shown, for example, in the top plan view of FIG. 2.Here, the conductive layer comprises a quadrangular region 12 a of anarea slightly greater than that of the chip, and connection regions 12b.

This type of attachment entails, however, problems. In fact, duringbonding, when the structure undergoes thermal treatments, for exampleduring the polymerization of the glue (carried out typically at 100-190°C.), since the air in the cavity 2 cannot exit and increases in volume,it exerts a pressure on the adhesive film 7 and the glue layer 8, whichare yielding. Swellings are thus created that tend to raise the chipand, in particular in presence of lack of uniformity, can cause tiltingof the chip 1 and delaminating of the glue layer.

The consequence thereof is that the yield of the assembly process islow, even lower than 50%.

The same problem applies to other types of MEMS devices, having cavitiesclosed by sealing layers of compliant material and/or bonded on areas ofcompliant material.

BRIEF SUMMARY

Some embodiments of the present disclosure provide a device and a methodthat overcome the drawbacks of the prior art.

According to some embodiments of the present disclosure, there areprovided an electronic device and the manufacturing process thereof, asdefined in claims 1 and 12, respectively.

In practice, in the support, underneath the area of the chip where thecavities are formed, there is a depressed portion containing part of theglue. During the thermal treatments, the glue, being compliant, allowsfor expansion of the air in the cavities within the depressed area, thuspreventing raising and/or tilting of the chip with respect to thesubstrate. Typically, the depressed portion comprises a recess formed bythe surface layers of the support. In addition, spacer elements mayextend within the glue layer so as to ensure the thickness of theswelling to be always smaller than the distance between the chip and theplane underlying the chip itself and to ensure planarity of the chip.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

For a better understanding of the present disclosure, preferredembodiments are now described, purely by way of non-limiting example,with reference to the attached drawings, wherein:

FIG. 1 is a cross-section through a chip having cavities and bonded to asubstrate;

FIG. 2 is a top plan view of the substrate of FIG. 1;

FIG. 3 shows the chip of FIG. 1 in case of tilting and delamination;

FIG. 4 shows a cross-section of the present device;

FIG. 5 is a top plan view of the device of FIG. 4;

FIG. 6 shows a cross-section, taken along section line VI-VI of FIG. 5;and

FIG. 7 shows an integrated chemical sensor for detecting odorousmatters, incorporating the present device.

DETAILED DESCRIPTION

FIGS. 4-6 show an electronic device 20 including a chip 21 bonded to asubstrate 22 by a glue layer 40. The chip 21 has cavities 23 extendingfrom the rear surface of the chip 21 and delimiting membranes 24 at thebottom. If the electronic device 20 forms part of a chemical sensor,adsorbent layers (not shown) may be formed on the membranes 24 and maybe of a material able to bind with the chemical substance to bedetected, as described in detail in aforementioned patent applicationU.S. patent application Ser. No. 12/648,996. For example, the adsorbentlayers can contain metal-porphyrins having an affinity with the chemicalmatters to be detected and form, together with the membranes 24, asensitive area 26. For the rest, the chip 21 may comprise a circuitryarea 25 including electronic components (shown schematically) so as toform, with the sensitive area 26, a device for detecting chemicalmatters, for example, odorous chemical matters.

An adhesive film 30 covers the rear surface 21 a of the chip 21 andcloses one or more cavities 23 at the bottom. The adhesive film 30 maybe a die-attach film, for example, of epoxy material having a thicknessof 10-50 μm, laminated on the rear surface 21 a.

The substrate 22 comprises a base layer 33 having a surface 33 a coveredby a conductive layer 34 and a protective layer 35, for instance ofinsulating material, arranged on top of the conductive layer. Forexample, the substrate 22 may be formed by a printed-circuit board, andin this case the base layer 33 may be a core layer, the conductive layer34 may be a metal material layer, such as copper, and the protectivelayer 35 may be a solder-mask layer. The core layer 33 is of an organicmaterial, for example BT (bismaleimide triazine), epoxy resin, FR-4(Flame Retardant 4), LCP (Liquid Crystal Polymer), or polyimide.

In the example shown, part of the conductive layer 34 and part of theprotective layer 35 are removed so as to form a recess 45 extending atleast underneath the area of the cavity or cavities 23, as shown in FIG.4, and having an area smaller than the area of the chip 21, in top planview. In practice, here the conductive layer 34 is shaped so as to form,i.e., an ring-shaped region 34 a and its width d_(e) (distance betweenthe inner and the outer edges of the ring) is such that the lateralsurface of the chip 21 falls, in top plan view, inside the ring-shapedregion 34 a. In other words, the external edge of the ring-shaped region34 a delimits a greater area than the area of the main face of the chip,in top plan view.

In the embodiment of FIG. 4, moreover, the glue layer 40 incorporatesspacer elements 41, for instance balls of insulating or conductivematerial. For example, the spacer elements 41 may be of a polymericmaterial, such as polytetrafluoroethylene (PTFE), or glass, metalmaterial, such as silver, and the like, and the glue may be of apolymeric material, such as an epoxy resin or a silicone material, or inany case of a softer material than the spacer elements 41, the support22, and the chip 21.

The spacer elements 41 preferably all have substantially the same presetthickness so as to ensure a plane rest for the chip 21.

In particular, by designating by A the depth of the recess 45 (sum ofthe thicknesses of the conductive layer 34 and of the protective layer35), by S the diameter or thickness of the spacer elements 41, by C theexpected thickness of the swelling that forms underneath the cavities 23because of thermal effect, and by B the distance between the bottomsurface 21 a of the chip 21 and the surface 33 a of the base layer 33(neglecting the thickness of the adhesive film 30), the dimensions arechosen so that:

B=A+S>C  (1)

Generally, with current manufacturing techniques, C is approximately 10%of the depth of the cavities 23. With cavities 23 having a depth of 500μm, C is thus normally comprised between 40 and 60 μm, and on average isapproximately 50 μm.

In case of a printed-circuit board, with a thickness of the copperconductive layer 34 of 15-20 μm and a thickness of the protective layer35 of 30-50 μm, the thickness A is typically comprised between 45 and 70μm, for example 50 μm.

To satisfy Eq. (1), it is thus sufficient to use spacer elements 41having a diameter of 25 μm. According to the application, materials, anddimensions of the various parts, the spacer elements 41 may, however,have typical thicknesses of 10, 15, 20, 25, 30, 50 μm or even 100 μm.

The recess 45 may be coupled to trenches 46 extending peripherally fromthe recess 45 throughout the width of the ring 34. In practice, as maybe seen in FIG. 6, in the area of the trenches 46 the conductive layer34 is removed. For example, the trenches 46 may have a width of 100-200μm and be arranged at a distance from one another of 300-500 μm.

In this way, during fabrication, after application of the glue on thechip 21 or on the substrate 20, also because the glue 40 is soft, it ispossible to cause any possible air trapped underneath the chip 21 toexit, thus eliminating a source of disturbance during bonding. Inpractice, the trenches 46 form venting channels.

The recess 45 and the trenches 46 may be formed while defining theconductive layer 34 and the protective layer 35, with a standardphotolithographic technique, in a simple and inexpensive way.Alternatively, they may be formed at the end of the manufacturing of thesubstrate 22, prior to bonding, using a specific milling operation. Inthis latter case, it is possible to remove just part of the conductivelayer 34 or, if the protective layer 35 is sufficiently deep, to removeonly the protective layer 35.

The device described herein has numerous advantages.

By virtue of a depressed area or recess underneath the chip 21, at leastin the area of the cavities 23, the air can expand during the thermalcycles and form swellings without causing raising of the chip from thenominal position. In fact, the chip always rests on a surface ofconstant thickness, guaranteed by the spacer elements 41. Any airpossibly trapped therein can exit from the trenches 46.

In this way, any tilting of the pads and delamination of the glue layeror of the film 30 are prevented, and a yield of the assembly processhigher than 80% is obtained.

Moreover, the recess 45 forms a side delimitation of the area subject todeformation, ensuring a good structural and conformational stability ofthe finished device.

Manufacturing and bonding may be carried out without additional costs ascompared to the device of FIGS. 1-3 if the recess is formed whilemanufacturing the substrate 22, by simply modifying the design of thestructures, without requiring specific operations during production ofthe substrate or in the assembly process.

The performances of the device are not affected by the assembly.

FIG. 7 shows an integrated chemical sensor 50 for detecting odorousmatters that may incorporate the device 20. The chemical sensor 50 isformed, for example, as described in U.S. patent application Ser. No.13/016,086 filed on Jan. 28, 2011, published as U.S. ApplicationPublication No. 2011/0209524, and incorporated herein by reference inits entirety. The chemical sensor 50 comprises a casing formed by a base51 and by a lid 52, enclosing the printed-circuit board that forms thesupport 22.

The lid 52 (represented in ghost view) has an input port 53 forintroduction of gases to be analyzed and defines a channel 54 extendingfrom the input port 53 to a suction fan 55, in turn connected to anoutput port 56.

The channel 54 extends on top of the chip 21, bonded on the side of thesupport 22 facing the lid 52 so that the gases entering the channel 54for the suction of the fan 55 lap the chip 21, and the odorous mattersto be recognized are captured in the sensitive area 26.

The support 22, on the side opposite the chip 21, carries othercomponents, such as, for example, a fan-control device 60, coupled viaconductors (not shown) to the fan 55, and an auxiliary chip 61, forexample a controller with memory, a signal-processing circuit, or thelike. In turn, the auxiliary chip 61 may be coupled to an externaldata-processing apparatus (not shown).

The base 51, which is to couple with the lid 52 so as to enclose thesupport 22 in between, has an input 63 and an output 64 for coolant air.

Finally, it is clear that modifications and variations may be made tothe device and the manufacturing process described and illustratedherein, without thereby departing from the scope of the presentdisclosure.

In particular, the depressed area may be formed in a surface layer ofthe substrate, creating a local recess or depressed area where a gluelayer can accumulate and which has a greater thickness than theneighboring areas so as to contain expansion and swelling of the air inthe cavities.

The spacer elements 41 may have a shape other than the spherical shape;they may, for example, be spheroidal, even irregular, with projections,for example shaped as flakes, but in any case preferably able to ensurea constant distance from the underlying layer of the chip, for exampleas a result of a preferential lie position that may be achieved duringapplication of the glue layer or by compression during application ofthe chip.

The conductive region 34 underneath the chip 21 may have a shapedifferent than the annular shape shown; for example, it may be formed byportions of any shape set alongside one another, or else delimit aplurality of recesses, one for each cavity 23 of the chip 21 or may evenbe absent.

Finally, the chemical sensor may be made differently, for example foranalysis of matters in a liquid, as described in U.S. patent applicationSer. No. 13/170,058, which published as U.S. Application Publication No.2011/0318840, is incorporated herein by reference in its entirety, anddiscloses a support that is not completely contained in the casing ofthe chemical sensor.

The various embodiments described above can be combined to providefurther embodiments. These and other changes can be made to theembodiments in light of the above-detailed description. In general, inthe following claims, the terms used should not be construed to limitthe claims to the specific embodiments disclosed in the specificationand the claims, but should be construed to include all possibleembodiments along with the full scope of equivalents to which suchclaims are entitled. Accordingly, the claims are not limited by thedisclosure.

1. An electronic MEMS device comprising: a chip having a main face, acavity extending inside the chip from the main face; a flexible filmcovering at least a portion of the main face of the chip adjacent to thecavity; a support; and an adhesive layer sandwiched between the flexiblefilm and the support for fixing the chip to the support, wherein thesupport has a depressed portion facing the cavity and a protrudingportion facing the main face of the chip and delimiting the depressedportion, wherein the adhesive layer has a greater thickness inside thedepressed portion than on the protruding portion.
 2. A device accordingto claim 1, wherein the support comprises: a base body having a surface,and a multi-layered structure formed on top of the surface of the basebody, wherein the depressed portion is formed by a recess in themulti-layered structure, the recess accommodating part of the adhesivelayer.
 3. A device according to claim 2, wherein: the support comprisesa printed circuit board; the base body includes a core layer; and themulti-layered structure comprises a shaped metal layer, extending on topof the surface of the base body, and a protective layer covering theshaped metal layer, the shaped metal layer and the protective layerforming the protruding portion delimiting the depressed portion.
 4. Adevice according to claim 3, wherein the shaped metal layer comprises agenerally ring-shaped region having an inner edge delimiting the recessand an outer edge delimiting an area that is bigger than the area of themain face of the chip.
 5. A device according to claim 4, wherein thegenerally ring-shaped region has a plurality of trenches or channelsextending between the inner and the outer edges.
 6. A device accordingto claim 1, further comprising spacer elements positioned in theadhesive layer.
 7. A device according to claim 6, wherein the adhesivelayer is of a softer material than the spacer elements, the support andthe chip.
 8. A device according to claim 6, wherein the adhesive layeris formed by an epoxy or silicone glue and the spacer elements are madeof a polymeric material, glass or metal.
 9. A device according to claim6, wherein the spacer elements have a spheroidal shape.
 10. A deviceaccording to claim 1, wherein: the support comprises: a base body havinga surface, and a multi-layered structure formed on top of the surface ofthe base body, wherein the depressed portion is formed by a recess inthe multi-layered structure, the recess accommodating part of theadhesive layer; the adhesive layer incorporates spacer elements; and thechip and the surface of the base body are separated by a distance givenby a sum of thicknesses of the multi-layered structure and of the spacerelements.
 11. A device for detecting odorous matters comprising: acasing having a channel for a gas to be analyzed and a motor foradvancing the gas to be analyzed; and an electronic MEMS device carriedby the casing, the MEMS device including: a chip having a main face andincluding a sensitive region of a material able to bind with targetchemicals in the gas to be analyzed; a cavity extending inside the chipfrom the main face; a flexible film covering at least a portion of themain face of the chip adjacent to the cavity; a support; and an adhesivelayer sandwiched between the flexible film and the support for fixingthe chip to the support, wherein the support has a depressed portionfacing the cavity and a protruding portion facing the main face of thechip and delimiting the depressed portion, wherein the adhesive layerhas a greater thickness inside the depressed portion than on theprotruding portion.
 12. A device according to claim 11, wherein thesupport comprises: a base body having a surface, and a multi-layeredstructure formed on top of the surface of the base body, wherein thedepressed portion is formed by a recess in the multi-layered structure,the recess accommodating part of the adhesive layer.
 13. A deviceaccording to claim 12, wherein: the support comprises a printed circuitboard; the base body includes a core layer; and the multi-layeredstructure comprises a shaped metal layer, extending on top of thesurface of the base body, and a protective layer covering the shapedmetal layer, the shaped metal layer and the protective layer forming theprotruding portion delimiting the depressed portion.
 14. A deviceaccording to claim 13, wherein the shaped metal layer comprises agenerally ring-shaped region having an inner edge delimiting the recessand an outer edge delimiting an area that is bigger than the area of themain face of the chip.
 15. A device according to claim 14, wherein thegenerally ring-shaped region has a plurality of trenches or channelsextending between the inner and the outer edges.
 16. A device accordingto claim 11, wherein the MEMS device includes spacer elements positionedin the adhesive layer.
 17. A device according to claim 16, wherein theadhesive layer is of a softer material than the spacer elements, thesupport and the chip.
 18. A device according to claim 16, wherein theadhesive layer is formed by an epoxy or silicone glue and the spacerelements are made of a polymeric material, glass or metal.
 19. A deviceaccording to claim 16, wherein the spacer elements have a spheroidalshape.
 20. A device according to claim 11, wherein: the supportcomprises: a base body having a surface, and a multi-layered structureformed on top of the surface of the base body, wherein the depressedportion is formed by a recess in the multi-layered structure, the recessaccommodating part of the adhesive layer; the adhesive layerincorporates spacer elements; and the chip and the surface of the basebody are separated by a distance given by a sum of thicknesses of themulti-layered structure and of the spacer elements
 21. A process,comprising: manufacturing an electronic MEMS device, the manufacturingincluding: forming a chip having a main face and a cavity extending fromthe main face; covering a portion of the main face of the chip, at leastat the cavity, with a flexible film; providing a support having adepressed portion and a protruding portion that delimits the depressedportion; applying an adhesive layer to the flexible film or to thesupport; and arranging the chip on the support so that the depressedportion faces the cavity and the protruding portion faces the main faceof the chip.
 22. A process according to claim 21, wherein providing thesupport comprises: forming a printed circuit board, including forming acore layer, forming a metal layer on top of the core layer, and forminga protective layer, covering the metal layer; and forming a recess byremoving selective portions of the metal layer and of the protectivelayer.
 23. A process according to claim 21, wherein the adhesive layeris formed by a glue incorporating spacer elements.